By Huygen's principle, an arbitrary optical field is fully determined once the light amplitude and phase are known over the beam aperture. Let that aperture be illuminated from the rear by a laser beam of known phase and amplitude. By introducing a thin-film optical element that adjusts the phase (by its optical thickness) and amplitude (by its transmittance) at each point across the aperture, it is possible to construct a totally arbitrary optical beam. This means that a uniform, collimated laser beam can be redirected and refocused to simultaneously illuminate multiple targets with adjustable intensity, phase-coherent radiation. This is, of course, the basis for all holography.
For a given illumination scene, the phase retardation and transmittance of the required holographic optical element can be computer calculated. Lithographic methods for fabricating computer generated holograms have been developed, and several realizations have appeared. The most common have been "binary" phase elements in which only 0.degree. and 180.degree. phase shifts are employed, although quaternary and octonary optical phase elements have been used or proposed. However, only very simple patterns have been produced, and placement errors in the patterning have proven troublesome.
It would be desirable to fabricate complex, computer generated, n-ary optical phase elements and to fabricate such elements with n-ary level amplitude programmed into discrete pixel locations, where n.gtoreq.16 but not necessarily the same for both amplitude and phase. Such an element could be produced using an electron beam lithography system 10 shown schematically in FIG. 1, such as a JEOL JBX-5DII, on a stage 12 controlled in orthogonal x and y directions by a stage controller 13 which receives digital x, y commands from a programmed control computer 14, such as a model PDP-11. The required control data is loaded from an off-line data preparation computer 15. An electron gun 16 is provided with the necessary restriction aperture, alignment coil, lens, blanking electrode and intensity control through a beam deflector 17. The e-beam deflector is provided with an alignment coil, astigmatism correction coil, aperture selector, scanners and lens.
The e-beam lithography system 10 relies primarily on the positioning of the stage 12 for exposing the resist 11 pixel by pixel in the pattern programmed into the computer 14 and secondarily on the beam deflector 17 for positioning the e-beam in increments less than one digital step of the controller 13. A laser measurement system (not shown) is used for precision measurement of the stage position at each step. That measurement is then employed by the control computer as the precision feedback necessary for exposure of the resist 11. The entire electron beam lithography system 10 is contained within an enclosure 18, and a constant vacuum is maintained within the enclosure by means not shown. The entire system is conventional. What is new is the method in which the resist is exposed pixel by pixel in n-ary levels with low doses and then partially developed to produce from the exposed and developed resist a hologram having n-ary phase delay of light transmitted through its exposed face. Such an electron beam holographic system may also be fabricated to provide both n-ary phase and n-ary amplitude holographic elements by providing a suitable mask applied to the unexposed face of the resist 11 in a novel manner.
For phase delay, the amount of exposure of the resist could be varied from pixel to pixel in accord with a calculated pattern. The resist thickness remaining after development would then determine the optical phase delay for each pixel. However, commercially available resists are optimized for high sensitivity, high contrast, and high resolution. For the n-ary optical phase, the resist should have low contrast, as will be noted more fully below, while for producing a mask for n-ary amplitude control of the phased delayed light, a separate resist used may have high contrast. As will be shown, both resists may be of the same type.
The JEOL electron beam lithography system is optimally used for exposing large areas, each at a fixed dose, but it may also be used to alter the exposure from pixel to pixel. To accomplish that, twelve bytes of information are needed for each pixel. Preparing and processing these multimegabyte information files can be expedited by writing a code to translate the pattern data directly into "scanner" format, bypassing several data conversion steps normally encountered using the JEOL system in a conventional manner. Writing a scanner code also enables the user to control the stage motion and eliminate positional errors caused by the direction of stage travel. The scanner coding may thus facilitate use of the JEOL system for n-ary (grey level) control in the fabrication of phase holograms (where n.gtoreq.16) of a size .gtoreq.1 cm square with an array of pixels, each pixel .ltoreq.3 .mu.m square, and placement accuracy better than .+-.50 nm for 3.sigma. (where .sigma. is a standard deviation) and accuracy of .+-.3%FS (full scale of maximum thickness) for 3.sigma..
Phase holograms thus produced may have numerous uses. They may, of course, replace the binary, quaternary and octonary optical phase elements now in use, or being proposed, with improved efficiency and greater capability. Such elements are used in a variety of image processing and pattern recognition applications. They are also used for both laser beam combining and beam splitting. In addition, the technology has direct application in integrated optics. For example, such elements may be used in various image processing and pattern recognition applications, as well as depth profiling an optical waveguide and optical beam steering, coupling or focusing.
Surface contouring an e-beam resist by controlling both the exposure dose and the development process has been demonstrated by H. Fujita, et al., Opt. Lett. Vol. 6, page 613 (1981); Vol. 7, page 578 (1982). They designed, fabricated and tested micro Fresnel-zone plates, blazed gratings and Fresnel lenses using as a resist polymethyl methacrylate (PMMA), normally a very high contrast material commonly known by the trademark Lucite. The exposure method used involved scanning the e-beam in either straight lines or circles with the dose adjusted to give the desired surface depth after development. This method produced somewhat irregular groove shapes, but efficiencies of 50%-60% were achieved with near-diffraction limited performance.
More recently M. Ekberg, et al., reported on kinoform (digitized) phase holograms in Opt. Lett., Vol. 15, pp. 568-569 (1990). These were patterns comprising a 512.times.512 array of 10 .mu.m square pixels, each with a unique high dose of an e-beam calculated to give the appropriate etch depth upon development. However, only ten levels of doses for levels of depth 20 nm level were used, and diffraction efficiencies of only 70% were reported.
High gamma (contrast) is a desirable property for photo- and e-beam resists used for device patterning. With high gamma, large variations in exposure dose will have little effect on the pattern shapes as long as the exposure is above a critical level. Hence, in the prior art, including that of Ekberg, et al., common resists and their development processes have been tailored in this direction of high contrast. However, high contrast does not allow for such precise definition of surface relief patterns as may be required in, for example, phase holograms with diffraction efficiencies significantly greater than 70%, which require greater precision in the etching of levels during development, particularly in levels much greater than 2 or even 10, such as 16 to 64 levels.